The manufacture of very fine particles, particularly extremely fine particles such as iron oxide, utilizes an electrostatic precipitator to remove the fine particles from the production stream. In the case of iron oxide particles, this means precipitating the particles, which are below about 100 .ANG. in size, from a moving stream of oxidizing gas. Generally, this is performed by passing the oxidizing gas and the particles through an ionizer (first stage) in which a direct current corona discharge electrically charges the particles. These charged particles then enter the collector (second stage) of the precipitator. The collector consists of a series of electrically charged, parallel, conductive plates which are polarized such that in general each plate of a given, constant polarity (positive or negative) is situated between two plates of opposite polarity. Each entering charged particle thus is attracted to, and adheres to, a collector plate of opposite polarity. The collected product is harvested periodically by physical removal from the collector assembly.
The problem with this process and particularly with the collection of fine iron oxide particles is that all commercially available, two-stage, electrostatic precipitators are designed for cleaning gas (usually air) streams containing only very low concentrations of particulate solids. After a long period of operation they are conventionally shut down, and the small amount of precipitated solids is removed, commonly by washing, an expensive process that would cause irreversible agglomeration and degradation of fine iron oxide particles. In a few applications in which very dense particles (such as those from arc-welding operations) are collected, in situ "rapping" (i.e., hammering or vibration) of the collector suffices for cleaning. Unfortunately the fine iron oxide particles manufactured by the process of interest are present in very high concentration in the gas stream, and they rapidly deposit on collector plates as thick, fluffy layers of high electrical resistivity and exceedingly low density. The thickness and high resistivity promote arcing between collector plates, a process that dislodges massive clots of the product back into the gas stream. These clots escape from the precipitator and thus are lost to the exhaust gases. For efficient recovery of the collected fine iron oxide particles it is necessary to shut down the precipitator frequently and thoroughly remove the precipitate. Rapping depends on inertial forces on the adherent precipitate. Because such forces are minimal in the case of the fine iron oxide particles, cleaning by rapping or vibration is very inefficient. Another disadvantage of rapping is the injuriously high noise level to which plant operators may be exposed.
Therefore, what is needed in this art is a cleaning method which is quiet, efficient, less time consuming and cost effective.